Heating, ventilation, air conditioning, and refrigeration (HVAC/R) systems generally operate according to AC power received from an AC utility power source, such as an AC mains. Accordingly, an HVAC/R system is generally shut down if the power source fails. When an HVAC/R system is used to cool food markets, restaurants, warehouses, cold storage lockers, etc. in which perishable goods such as fresh produce, dairy products, and meats are held, or pharmacies, hospitals, and other medical facilities where temperature sensitive medicines, drugs and the like are held, shutting down the system may result in spoilage of the goods and significant economic loss.